I think (I hope) that this should not be a problem.

Indeed, first, I thought you needed to restore a db but that your server
was still up and running to do the move media and the checkin.
Obviously, this is not the case.

Anyway, the restore db (a the DOS prompt) is just using the volhist file
to know what is the last available Database Backup.

I am almost sure that your last database backup, on-site or off-site is
declared in the volhist file.
run DSMSERV DISPLAY DBVOLUMES to verify if TSM is aware about it.

So, using restore db will not take care about the fact that the tape is
on-site or off-site but will just send a request to the library to read
it.
SO, just insert your tape in the library and type :

dsmserv restore db todate=xx:xx:xxxx totime=xx:xx

or, if the tape is unknown in your volhist file:

dsmserv restore db volumenames=yyyy todate=xx:xx:xxxx totime=xx:xx

Ruddy